Factor VII and haemostasis.

The role of factor VII in the haemostatic mechanism as well as thrombosis has recently gained new interest. Today's concept that factor VII may be a key regulator in the initiation of blood coagulation is based on studies that provide new evidence for a mandatory activation of factor VII to factor VIIa in blood. Exposure of thromboplastin to the circulation may not trigger activation of blood coagulation before the one chain factor VII is converted to the active two chain form of factor VIIa. A hypothetical model is proposed for the initiation and subsequent activation steps of the blood coagulation process. In this model, it is suggested that circulating activators of factor VII activate inactive complexes of thromboplastin-factor VII. Subsequently, newly generated factor Xa will accelerate this reaction and thereafter be the most potent activator of factor VII. This model would also fit with the clinical observation that moderate factor VII deficiency may be associated with thrombotic episodes discussed in this communication. This article also discusses the role of recombinant factor VIIa in the treatment of factor VIII deficiency patients with acquired factor VIII inhibitors, factor VII and ischemic heart disease and the factor VII-phospholipid complex, and the regulation of the thromboplastin-factor VIIa complex by factor Xa and extrinsic pathway inhibitor (EPI).